Cellular recognition of the extracellular matrix (“ECM”) proteins and of other cells has a complex molecular basis, involving multiple distinct cell surface receptors. Integrins are a family of receptors that are fundamentally important for mediating cell adhesion to ECM proteins. Tumor cells adhere to variety of ECM proteins and molecules on other cells as they invade and metastasize. These interactions of tumor cells have a profound effect on their phenotype. Although its exact role is complex and not completely understood, α4β1 integrin has been implicated in tumor cell arrest and/or extravasation and is involved in tumor cell invasion and metastasis. This integrin is expressed on many hematopoietic malignancies and also on tumors such as melanomas. α4β1 integrin is unique among integrins in that it binds to both ECM components (e.g. fibronectin) and Ig superfamily adhesion receptors (e.g., VCAM-1) which are expressed on activated endothelial cells and other cell types. α4β1 integrin also binds to itself and promotes homotypic cell adhesion. Although a role for α4β1 integrin has been established in modulating various aspects of tumor cell biology, the mechanisms by which the function of the α4β1 integrin is modulated are complex and not well understood. Understanding the nature of such interactions may help to explain cell-type specific behavior on ECM proteins that are often observed with integrins. There is, accordingly, a continuing need to identify peptides capable of modulating α4β1 dependent cell adhesion as a means of furthering the understanding of the complex interactions involving this integrin.